1. Field of the Invention
The present invention relates to a flat-type cathode ray tube (CRT) and more particularly to an improvement of a curvature of a panel for the CRT.
2. Description of the Related Art
In general, the CRT is an image display device for displaying a picture signal received thereto as an image and is divided into a curved type having a predetermined curvature in both the inner surface and the outer surface of the panel and a flat type wherein the inner surface of the panel has a predetermined curvature, while the outer surface thereof is substantially flat.
FIG. 1 shows a sectional view of a conventional curved-type CRT, and FIG. 2 shows an exploded perspective view of a shadow mask and a frame constituting the curved-type CRT of FIG. 1. FIG. 3a shows a top view of a panel assembly having a show mask and an inner shield combined therewith and FIG. 3b shows an operational view of the panel assembly.
The curved-type CRT used in general will be explained with reference to the associated drawings.
As shown in FIG. 1, the conventional curved-type CRT comprises a rectangular panel of which outer surface has a predetermined curvature, a nude-type funnel and an electron gun assembly.
More specifically, the panel 1 comprises a phosphor screen 4 formed by depositing red, green and blue phosphors on the inner face of the panel 1 and a shadow mask 3 which is installed on the inner surface of the panel 1 at a predetermined space relationship therewith and which has a rectangular frame 7 fixed to the rear side thereof. Also, an inner shield plate 9 is attached to the rear side of the frame 7. A funnel 2 comprises a neck portion 110 into which an electron gun assembly for emitting electron beams 6 based on received picture signals is inserted.
Further, as shown in FIG. 2, the shadow mask 3 comprises a number of fine apertures 31 for passing through electron beams emitted from the electron gun and its whole skirt portion is welded on the inner wall or outer wall of the frame 7 by means of a welding process.
Also, the frame 7 includes a flat portion in parallel to its short axis direction in every edge of the outer wall thereof and has an elastic supporting member, spring 8, fixed to one side of the flat portion.
The spring 8 has a first arm portion and a second arm portion being contacted at a predetermined angle each other, wherein the first arm portion has a hole on its surface (not shown) to be held on a stud pin of the frame and the second arm portion is fixed to the flat portion by welding.
As stated above, by installing the frame 7 having the shadow mask 3 fixed thereto onto the inner surface of the panel 1 and the inner shield plate 9 on the rear side of the frame 7, a panel assembly is formed as shown in FIG. 3a. 
Especially, the panel 1 has the radius of curvature defined by a level difference between the center of the inner surface of the panel 1 and the outer end of effective area thereof, and comprises stud pins 10 which are anchored on two or three edge portions of the inner wall of the panel 1.
The process for displaying picture through a conventional CRT having such construction will be described hereafter.
When image signals are input to the electron gun, electron beams 6 as the image signals are emitted from the gun to the phosphor screen 4. The electron beams 6 emitted are deflected by deflection york, pass through a number of apertures 31 of the shadow mask and radiate red, green and blue phosphors on the phosphor screen deposited the inner surface of panel 1 thereby to display the picture.
At this time, about 20% of the electron beams pass through apertures 31 of the shadow mask and the rest 80% collides with the shadow mask, resulting in the rising in temperature of the shadow mask and frame.
Thus, in the shadow mask comprised of AK material or invar alloy having a low thermal expansion coefficient and the frame comprised of iron having a high thermal expansion coefficient, the frame is expanded during the early rising in temperature and then the shadow mask 3 is also expanded as time passes by.
Referring to the operation diagram of FIG. 3b, the dotted line shows a shape of the shadow mask 3 and the frame 7 of the curved-type CRT before operation, while the solid line shows a shape of the shadow mask 3 and the frame 7 of the CRT, wherein the position of the shadow mask 3 and the frame 7 are shifted due to the rising in temperature caused by electron beams in operation.
That is, the shadow mask 3 and the frame 7, which were increased in volume due to their temperature rising, are shifted toward phosphor screen. Such shift of the shadow mask and the frame can be recompensed by a repelling power of the spring 8 as an elastic supporting member, however, it is difficult for the shadow mask to be returned to its position before operation.
Therefore, when electron beams are scanned onto phosphors on the phosphor screen in the inner surface of the panel, an amount of variation (xcex94LE) in landing of electron beam at the phosphors (referred to as xe2x80x9cmislanding quantity (xcex94LE)xe2x80x9d hereinafter) appears because of the position change of the apertures 31 caused by the shift of the shadow mask 3.
Also, when the impact energy generated by external impact or a sound pressure of speaker arrives at the frame 7 and the shadow mask 3 through the stud pin 10 and the spring 8, so-called doming phenomenon which causes a tremble of the shadow mask occurs. As a result, the position of the apertures 31 is changed and the mislanding quantity (xcex94LE) of the electron beam occurs, thereby deteriorating transmissivity of electron beams and the brightness of a picture and lowering the color purity of the picture with the incorrect beam arrangement.
Further, because of a low height of the frame in the short axis direction due to the curved shape of panel, the frame does not fully wrap the outer portion of the shadow mask, thereby causing a significant deformation of the shadow mask in manufacturing it or an occurrence of black stripes on the screen due to the mislanding of the electron beam.
In addition, since the inner shield plate attached on the rear side of the frame so as to prevent magnetic field of the earth magnetism has also a smooth plane shape on both the long peripheral portion and the short peripheral portion, a magnetic flux is not easily concentrated onto the long peripheral portion, the short peripheral portion as well as the corner portion. As a result, the magnetic field of the earth magnetism is not shielded effectively and thus the electron beam is deflected, resulting in occurrence of mislanding quantity (xcex94LE) of the electron beam and deterioration of the color purity of picture.
In order to solve the above mentioned problems, it is the object of the present invention to improve the brightness and color purity of picture by reducing the mislanding of the electron beam, thereby preventing doming, howling, magnetic field of the earth magnetism and the like.
To do this, the present invention provides a flat-type cathode ray tube comprising a panel for displaying a picture, wherein the outer surface of the panel is plane and the inner surface thereof has inner surface curvatures respectively along the vertical axis direction and the horizontal axis direction.